Method and system for closed loop combustion control of a lean-burn reciprocating engine using ionization detection

ABSTRACT

A system and method for controlling knock in a lean burn internal combustion (IC) engine includes a spark plug having an electrode, and an electrical circuit configured to provide a first voltage to the electrode and detect an ion current during a thermal-ionization phase of the combustion process, and provide a second voltage to the electrode to create a spark and initiate a combustion process within a combustion chamber. The engine includes a controller configured to monitor the ion current for a knock condition that includes at least an incipient knock condition, determine a spark crank angle timing of the IC engine where the incipient knock occurs, and adjust the spark timing of the IC engine to operate at a crank angle that does not exceed a threshold level beyond an incipient knock set point.

CROSS REFERENCE TO RELATED APPLICATION

The application is a continuation-in-part of and claims priority topatent application Ser. No. 11/864,820 filed Sep. 28, 2007, which claimsthe benefit of U.S. Provisional Application Ser. No. 60/973,276, filedSep. 18, 2007, and U.S. Provisional Application Ser. No. 60/827,364,filed Sep. 28, 2006.

BACKGROUND OF THE INVENTION

The present invention relates generally to spark-ignition engines, andmore particularly, to optimizing lean-burn operation with the use ofionization detection in a natural gas lean-burn engine and preventingengine knock by accurately detecting incipient knock and controllingengine operation in response thereto.

Spark-ignition (SI) reciprocating engines typically have a feasibleoperating regime determined by air-fuel ratio and spark timing. Suchengines typically operate with a safety margin of 0.5% O2, or with a 5-6degree timing margin. This is commonly referred to as knock margin.Conventional spark-ignition engines typically operate near thestoichiometric air/fuel ratio and depend upon exhaust after treatmentwith catalytic converters to reduce the nitrogen oxide (NOx) emissions.With increased emissions standards in the recent years, the industry ismoving toward lean-burn operation despite the difficulty of maintaininga stable combustion process in such engines due to a relatively largecoefficient of variation (COV). By running lean (i.e. operating with anair-fuel ratio greater than 1.4), turbocharged engines can enhance fuelefficiency without sacrificing power while producing less NOx pollutantsthan conventional stoichiometric engine operation. However, suchoperation is limited by engine knock which typically occurs duringlean-burn operation. In order to obtain a maximum power and optimizedfuel economy for lean-burn operation, it is desirable to detect theonset of engine knock and to operate near the knock limit (e.g. withreduced knock margin) without damaging the engine.

Accelerometer-based knock sensors are commonly used for detecting knockin SI engines. Accelerometers are mounted to the engine block to detectthe high frequency vibrations generated during knocking. However, theyare highly susceptible to electrical noise, and knock sensing can becompromised by engine mechanical noises like vibrations during valveclosure or piston slapping, especially at high engine speeds. Thus, thesignal typically must be filtered, reducing the overall sensitivity ofthe sensor and hindering such sensors from detecting incipient knock.Incipient knock is defined as a miniscule knock that does not contain aknock frequency that is adverse to engine operation. In essence, sensingincipient knock as an indicator of impending knock production would beuseful in controlling engine operation and avoiding knock all together.

In-cylinder pressure sensors have been used to provide directinformation about the intensity of knock, which makes them more valuablefor knock detection than accelerometers. However, due to the high costof these sensors and the costs associated with setup and operationthereof, they are used mainly in laboratory settings and are notpractical for high-volume field applications.

In-cylinder ion sensors have been used in recent years as a lower costalternative to the abovementioned knock sensors. They provide a directmeasure of in-cylinder thermodynamic conditions and can provideinformation about knock intensity. However, in lean-burn operation,because of the lean nature of the mixture, the ionized speciesconcentration is much less than at stoichiometric conditions. Thus,integrating the signal cannot be done reliably due to a number offactors that include high levels of noise relative to the ion signalmagnitude, variability of the ion signal, and low magnitudes of aresultant integrated signal. An ion sensor in a lean-burn engine alsotends to exhibit great variability, typically due to changes in fuelcontent, temperature, and humidity. However, these systems are also notsensitive enough to detect the onset of incipient knock. For example, aknock detection system employing knock frequency measurement will onlydetect strong detonation, not incipient knock, as described above, whichcontains virtually no spectral content.

Thus, the techniques developed using ion sensors for stoichiometricoperation are unsuitable for lean-burn operation and previous knockdetection systems have been limited thereto.

It would therefore be desirable to have a system and method capablereliably and affordably detecting incipient knock in a SI engine andcontrol operation of the engine to avoid entering into a frequencyproducing knock condition.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to a system and method of detectingincipient knock in a lean-burn reciprocating SI engine. An electricalcircuit controls an igniter or spark plug for ignition and for detectingan ionization signal within a combustion chamber of the SI engine thatis indicative of incipient knock. The incipient knock signal is used ina closed loop ignition timing control to maintain the engine at optimalefficiency devoid of any knock frequency.

In accordance with one aspect of the invention, a system for controllingknock in a lean burn internal combustion (IC) engine includes a sparkplug having an electrode, and an electrical circuit configured toprovide a first voltage to the electrode and detect an ion currentduring a thermal-ionization phase of the combustion process, and providea second voltage to the electrode to create a spark and initiate acombustion process within a combustion chamber. The engine includes acontroller configured to monitor the ion current for a knock conditionthat includes at least an incipient knock condition, determine a sparkcrank angle timing of the IC engine where the incipient knock occurs,and adjust the spark timing of the IC engine to operate at a crank anglethat does not exceed a threshold level beyond an incipient knock setpoint.

In accordance with another aspect of the invention, a method forcombustion feedback control of a lean-burn reciprocating internalcombustion engine using ion signals includes the steps of positioning aspark plug having an electrode, the spark plug positioned at leastpartially within a combustion chamber of the engine, initiatingcombustion within the combustion chamber by providing a voltage to theelectrode, measuring an ion current using the electrode duringcombustion, and adjusting spark timing of the IC engine to achieve andmaintain maximum thermal efficiency by operating at reduced knockmargin.

In accordance with yet another aspect of the present invention, aclosed-loop controller for a spark-ignition internal combustion (IC)engine includes a control to detect an ion current within a combustionchamber of the IC engine using an electrode of a spark plug, determine adesired crank angle for spark timing from the ion current whereinincipient knock begins to occur, and continually monitor and adjustspark timing of the IC engine to operate at or below inception ofincipient knock.

In accordance with still another aspect of the present invention, asystem for controlling knock in a lean burn internal combustion (IC)engine includes an electrical circuit and a controller. The electricalcircuit is configured to provide a first voltage to an igniter anddetect an ion current during a thermal-ionization phase of thecombustion process, and provide a second voltage to the igniter tocreate a spark and initiate a combustion process within a combustionchamber. The controller is configured to monitor the ion current for aknock condition that includes at least an incipient knock condition,determine the spark crank angle of the IC engine where the incipientknock occurs, and adjust spark timing of the IC engine to operate at acrank angle that does not exceed a threshold level beyond inception ofincipient knock.

Various other features and advantages of the present invention will bemade apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate embodiments presently contemplated for carryingout the invention.

In the drawings:

FIG. 1 is a high-level structure of a control system according to anembodiment of the present invention.

FIG. 2 illustrates a reciprocating engine having an ignition anddiagnosis module according to an embodiment of the present invention.

FIG. 3 is an electrical circuit providing spark voltage and ionmeasurement bias according to an embodiment of the present invention.

FIG. 4 is an illustration of a combustion process sensed by a pressuregage and an ion sensor according to an embodiment of the presentinvention.

FIG. 5 is an illustration of a post-flame ion signal versus crank anglefor various spark timing crank angles and relative propensity fordetonation.

FIG. 6 illustrates a combustion chamber having a shielded spark plugaccording to an embodiment of the present invention.

FIG. 7 illustrates an overview of an incipient knock control algorithmaccording to an embodiment of the present invention.

FIG. 8 illustrates an overview of an algorithm for controlling incipientknock according to an embodiment of the present invention.

FIG. 9 illustrates an ion signal peak location evaluation algorithmaccording to an embodiment of the present invention.

FIG. 10 illustrates an event counting algorithm according to anembodiment of the present invention.

FIG. 11 illustrates an ignition controller response to combustionparameters according to an embodiment of the present invention.

FIG. 12 illustrates an incipient knock algorithm and engine adjustmentparameters according to an embodiment of the present invention.

FIG. 13 illustrates an incipient knock algorithm to retard or advanceignition setpoint according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A system and method of detecting incipient knock in a lean-burnreciprocating SI engine is shown to reduce knock and operate at animproved thermal efficiency. An electrical circuit controls the sparkplug for ignition and for detecting an ionization signal within acombustion chamber of the SI engine that is indicative of incipientknock. The incipient knock signal is used in a closed loop ignitiontiming control to maintain the engine at optimal efficiency devoid ofany knock frequency.

Advancements in sensor and spark plug technology along with improvedalgorithms, described herein, has made it possible to obtain goodquality, reliable ion signals in very lean-burn engines. Enginestypically have a map of acceptable spark timings over all speeds andloads within an operating range. These timing maps are typicallycalibrated to avoid the knocking region by having a predetermined marginof safety.

Knock prevention and control using ion sensors can extend the availablesafe operating regime of an engine. The ion knock sensor thus allows theengine to be operated at a more advanced ignition timing, which istypically at a higher thermal efficiency.

The present invention relates to a system that will detect and procession signal in a lean-burn reciprocating engine to perform various enginediagnostics, such as detecting the onset of incipient knock. For allpurposes used herein, a lean-burn reciprocating engine is defined as anengine that operates having an air/fuel ratio greater than 1.4. Thisdiagnosis can then be used in a closed loop control to allow the engineto be operated at higher efficiency by operating at a lower knockmargin. This system can also be alternatively used for closed loopignition timing control to maintain an engine timing set point between aconservative base timing up to maximum advance for highest efficiency.The algorithms disclosed herein can be used alone in a knock preventionsystem, or in combination with other engine diagnostic and controlsystems.

FIG. 1 shows a high-level structure of a control system 100 connecting adiagnosis module 102 and a control module 104 according to an embodimentof the invention. In general, during operation, combustion feedback 106is fed into the engine diagnosis module 102 where it is diagnosed at108. Based on the diagnosis parameters 108, engine control module 104issues control commands 110 to adjust engine operation accordingly. Thediagnosis module 102 can function separately with its ownmicroprocessors or be integrated to be a part of the engine controlmodule 104 having other inputs and outputs. Alternatively, the enginecontrol module 104 can include for instance an ion signal analyzer,which receives the ion signal from the diagnosis module 102 anddetermines if knocking exists. In a preferred embodiment, the enginediagnosis module 102 provides an indication of knock to the enginecontrol module 104, which may then determine what action to takeaccording to embodiments of the present invention.

FIG. 2 illustrates an application of system 100 of FIG. 1 in relation toa spark ignition (SI) reciprocating engine 210 according to anembodiment of the present invention. System 200 includes a diagnosismodule 202 and an ignition module 204 which are controlled by an enginecontrol module 206. The diagnosis module 202, in this embodiment, sensesion current which occurs in a gap of a spark plug 212. These modules202, 204, 206 can function alone or be combined into a single module, orthey can be part of an engine controller having other inputs andoutputs. The reciprocating engine 210 includes an engine cylinder 214, apiston 216, an intake valve 218 and an exhaust valve 220.

In a typical four-stroke spark-ignited engine, the operation can bedivided into four cycles: intake, compression, expansion and exhaust.Near the end of the compression cycle, referring still to FIG. 2, thespark plug 212 ignites an air/fuel mixture contained in the cylinder 214at 222, which initiates combustion 224. During combustion, free ions aregenerated which are electrically conductive and can be measured byapplying a voltage across an ion probe. Because a spark plug works wellas an ion probe, a voltage may be applied across the gap of the sparkplug 212 to measure an ion current within the combustion cylinder 214during a combustion event. Using the spark plug 212 as the ion probeeliminates the need for an additional sensor in the cylinder. The ioncurrent may be measured in the engine diagnosis module 202 and containscircuitry for detecting and analyzing the ion signal. In a preferredembodiment, the diagnosis module 202 supplies power to the spark plug212 after the air and fuel mixture is ignited, and also measures ionsignals that occur in the spark gap. Alternatively a conventional ionprobe or other conventional device to detect ions may be used to measurethe ion signals that occur during combustion.

The following terms and definitions are applicable in this document forpurposes of clarity:

Normal engine operation. At a normal or conservative ignition timing incylinder measurements would show some normal distribution or sigma ofion and pressure peak locations and magnitudes over a number ofcombustion cycles. This distribution varies somewhat within some typicalrange for a given engine and operating condition. In general these areindicators of the repeatability of combustion from cycle to cycle.

Incipient knock. FIG. 5 shows a multi-cycle average of N post-flame ionwaveforms versus crank angle for various spark timing angles andrelative knock margins, where N is a number of combustion cycles. Asspark timing is advanced from a normal or conservative setting (forexample 20 degrees), peak pressure and peak ion distributions may varyonly slightly from the peak location distribution of N combustioncycles. However, at the onset of knock (or incipiency) the peak locationdistribution, or sigma, increases measurably and more specifically maybecome skewed toward advancing crank angle degrees i.e. (anon-symmetrical distribution). Furthermore, as shown in FIG. 5 an ion(or pressure see FIG. 4) waveform starts to show signs of more rapid (oruncharacteristic) pressure rise as incipiency is approached. In thecombustion process, knock occurs as the in-cylinder pressure andtemperature increases such that after the initiation of combustion bythe spark, a spontaneous second ignition takes place due to the increasein pressure and temperature. For the purpose of the ongoing discussion,incipient knock is defined as a small secondary spontaneous ignitionresulting in an increase in the distribution of pressure and ion peaklocations that does not result in a knock frequency. Knock frequency iscommonly understood within the art. For incipient knock the secondaryspontaneous ignition results in a small pressure wave traveling throughthe combustion chamber, but not of sufficient magnitude to be reflectedby combustion chamber surfaces resulting in pressure oscillations. Thissecondary spontaneous ignition also results in more rapid combustionduring a combustion cycle with incipient knock, thereby skewing the peaklocation distribution toward advancing crank angle degrees. This skeweddistribution can be used to detect incipient knock with either simple orcomplex algorithms. For example a single advanced peak when compared tothe normal peak location distribution of N combustion cycles canindicate an incipient knock event.

Classic, moderate, and hard knock. These refer to the presence of knockfrequency content in the ion or pressure waveform as is commonlyunderstood within the art. The knock frequency corresponds to theresonant frequency of the combustion chamber at the given in-cylinderconditions i.e. (pressure). Knock frequency is typically observed whenthe secondary spontaneous ignition is of sufficient magnitude to bereflected by combustion chamber surfaces and result in pressureoscillations Most knock detection systems require a minimum of severalcycles of knock frequency to be present for valid knock detection.

Ion currents measured in lean burn engines are typically on the order oftens of nano-amps. Circuitry for measuring ion currents within an ICengine, however, has been typically developed for stoichiometric engineoperation and is not capable of accurately measuring such signal levelsduring lean burn operation. In addition, the number of cylinders isoften greater than typical stoichiometric engines. This high cylindercount can cause the timing of the sparking cylinders and ion sensingcylinders to overlap, thus causing electromagnetic interference (EMI) orcross-talk to occur in the ion sensing circuit when the spark plugs arefired in a neighboring cylinder or cylinders. Ignition coil primarydrive currents are typically in the range of 30 to 40 amps. On thesecondary of the coil, very short duration currents associated with thespark breakdown can be on the order of 100 amps or more. The resultingarc current forms in about a nanosecond, and in this very short time thespark plug gap voltage drops from roughly 30 to 40 Kilovolts to lessthan 100 volts. This very large and fast current edge creates anothersource of strong EMI. Furthermore, noise present on the combustion ionwaveform can cause combustion detection algorithms to produce erroneousresults. Thus the development of an ion sensing system forhigh-cylinder-count lean burn engines presents considerable challenges.

Referring to FIGS. 2 and 3, a capacitive discharge ignition (CDI) isillustrated that charges one or more large capacitors 302 to typically300 volts. As is customary, the engine control module 206 receivesengine information through several sensors, the most important of whichis a crank angle position detector. This detector supplies pulses to theignition module 204 and engine control module 206 at specific crankangle increments. The control modules 204, 206 can therefore preciselytime control actions to the present engine crank angle. The ignitionmodule 204 can thereby fire the cylinder spark plugs (i.e. 212 of FIG.2) at the desired timing for correct engine operation. When the firingtime of a given cylinder is determined, the 300 volt charge on thecapacitor 302 is connected to the primary 304 of an ignition coil 306.The ignition coil 306, or step up transformer, increases the voltage toaround 40 kilovolts, which is sufficient to cause the spark gap 320 tobreakdown or start to arc. The arc ignites the fuel and air mixturecausing the desired combustion process. The ignition module 204 cyclesthrough all cylinders in the engine, firing them in the proper order andthen repeats the sequence. Shortly after the spark duration is complete,the ion bias energy source 308 is energized to commence the process ofmeasuring the ion current for the cylinder just fired. At this point theignition module 204 is preparing to fire the next cylinder in the firingorder. The diagnosis module 202 commences operation of bias energysource and the ion detection circuits for the cylinder just ignited.

A capacitor 312 within the ion detection circuits is recharged toprovide the desired ion bias voltage. Once an ion bias is present on thespark gap 320, ion current measurement can commence. The diagnosismodule 202 senses the ion current, amplifies it and digitizes it forprocessing. If an incipient knock is detected on a cylinder, thediagnosis module will report the knock to the engine control module 206after the completion of the combustion cycle for that cylinder. Oneskilled in the art would recognize that, although the diagnosis module202 may control separate bias supplies and ion detection circuits forall cylinders, corrective algorithms may be performed by a common singleprocessor or several processors.

The engine control module 206 receives inputs from various enginesensors and other plant systems and maintains desired engine operatingparameters. Upon receiving a combustion feedback parameter from thediagnosis module 202, the engine control module 206 determines thecorrect control response. For example when it receives indication ofincipient knock detected in a cylinder, the engine control module 206may determine the proper control action is to retard the timing by, forinstance, 0.5 degrees. It would then signal the ignition module 204 toretard the timing on the cylinder by 0.5 degrees. One skilled in the artwould recognize that other control responses are also possible. Forexample if several knocking engine cycles have been detected and severalattempts to retard the timing have failed to prevent the knock, theengine control module 206 may shut down the engine.

FIG. 3 illustrates a detailed view of the diagnosis module 202 of FIG. 2according to an embodiment of the present invention. A first powersource 302 is shown connected to the primary windings 304 of an ignitioncoil 306. A second power source 308 is shown as the ion bias rechargesupply. On command from a processor 310, the second power source 308recharges a capacitor 312 within the coil 306 to, for instance, 400volts. The second power source 308 turns off and the capacitor 312supplies a bias to the ion current loop formed by the capacitor 312, azener diode 314, a secondary winding in the coil 316, one or more highvoltage diodes 318, and a spark gap 320. The bias thereby enhances ioncurrent flow through the spark gap 320 if combustion is present. The ioncurrent passes through an amplifier 322. The ion signal proceeds througha low pass filter 326. This is typically a second or third order 20 kHzlow pass filter.

It is generally desirable to allow knock frequency to pass through tothe processor 310 if present during more severe knock conditions. Knockfrequencies are generally in the range of 2 to 6 KHz, but may alsocontain additional harmonics at two to three times this frequency.Therefore the filter characteristics of filter 326 are chosen toattenuate, typically, greater than 20 kHz signals, which tend to benoise. Thus, the low pass filter 326 may prevent noise from beingfalsely detected as knock or other combustion characteristics.

Referring still to FIG. 3, a multiplexer 328 is illustrated whichtypically switches to the next cylinder in the firing order when 1) thecombustion has been initiated in a cylinder, 2) the bias recharge eventhas taken place, and 3) it is time to start digitizing the ion currentwaveform. For example, in a 16 cylinder engine, the multiplexer 328would switch from one cylinder to the next in sequence with the enginefiring order. In some cases where the cylinders are closely spaced, itmay be desirable to utilize the two-bank architecture in the diagnosismodule 202 as illustrated in FIG. 2. That is, there may be onemultiplexer 328, an analog to digital converter (ADC) 330, and processor310 for each bank providing the capability for simultaneously processingto two ion waveforms.

An ADC 330 samples the filtered and processed ion current waveform andconverts it to digital format for use by the processor 310. The ADC 330typically samples the ion current waveform at a high enough rate suchthat the true characteristics of the sampled waveform can be evaluatedby control algorithms, or if necessary the ion waveform can bereproduced in real time by a digital to analog converter (DAC) (notshown). The sample rate is typically 50 kHz. One skilled in the artwould recognize that the multiplexer 328 and ADC 330 may be included asa subsection of the processor 310, and would not require separatedevices as illustrated.

The processor 310 receives the sampled ion data from the ADC 330 andexecutes the detection and evaluation algorithms according toembodiments of the present invention. The processor 310 is typically adigital signal processor (DSP) specifically designed to process largequantities of sampled data. The processor 310 may use internal orexternal memory in the course of performing the algorithms. Randomaccess memory (RAM) is typically required to perform filtering oraveraging of waveforms over many cycles of the engine. The processor 310sends combustion feedback parameters to an engine control module, suchas the engine control module 206 of FIG. 2. These parameters may includean incipient knock detection message for a specific cylinder. Otherparameters may include ion peak location or knock intensity i.e.(incipient, moderate, heavy, or shutdown).

One skilled in the art would recognize that high cylinder countlean-burn engines can have as many as 24 cylinders or more. Accordingly,although 16 cylinders were referenced for purposes of illustration, theignition, ion current sensing, and algorithms illustrated herein may beapplicable to 24 cylinders or more. Furthermore, as an example, thesystem illustrated contains two DSPs with integrated multiplexers andADCs thereby allowing simultaneous processing of cylinders on bothengine banks.

FIG. 4 is an illustration 350 of the sequence of events, as a functionof crank angle, that produces the ion signal 352 from a spark plug in atypical reciprocating engine. Based on the ion signal 352, the beginningof the combustion is clearly shown by a first peak 354, which is theignition pulse or start-of-spark phase. Immediately following the spark,a flame kernel starts to form and grows between the spark plugelectrodes. The proximity of the initial flame kernel to the ion probegenerates significant ion flow, thus leading to the chemi-ion phase at356. The peak 356 appearing in this phase of the ion signal 352 is thepoint where the ion formation equals the ion recombination.

Once the flame kernel leaves the gap, the flame finishes its earlydevelopment and the ion signal 352 continues to decline due torecombination reactions. However, during the end of the compressionstroke, the burned gas remaining in the vicinity of the spark plug gapis compressed by the moving flame front and moves back toward the sparkplug, which results in a higher gas temperature around the spark gap.After a period of decline, the ion signal 352 starts to rise again whenthe ion formation rate becomes stronger than the ion recombination rate.This also signifies that the reacting products remaining between thegap, which already have a very high temperature, are ionized again dueto the temperature increase resulting from the compression. This newpeak 358 is called the post-flame phase or the thermal-ionization phase.This post-flame phase 358 is related with the temperature and pressurerise and is the key phase in the ion signal 352. A correspondingpressure curve 360 correlates to the ion signal thermal ionizationphase, and may indicate the presence of oscillations or disturbances ifknock is present. The post-flame phase 358 is crucial to collectingdetonation information during knocking of the engine. After thepost-flame peak 358, the ion formation declines rapidly, resulting in anindication of the end of combustion and the complete loss of ion signal352. The process 350 then repeats for subsequent combustion cycles.

At lean fuel conditions, flame temperatures are typically not as highand post-flame ion peaks, such as peak 358 of FIG. 4, are not easilydetected using previous-generation ion sensors. This has prevented flameion sensing from being used to any significant extent in lean-burnengines. Moreover, in lean burn engines, the spark plug design, inconjunction with the gas-dynamic and thermodynamic characteristics ofthe combustion event, greatly affects the magnitude and repeatability ofan ion signal. For example, on one hand, systems having spark plugs witha high electrode surface area and electrodes, which are mostly shieldedfrom the combustion chamber flow, provide higher magnitude and moreconsistent ion signals than other types of spark plugs. On the otherhand, an ion signal is not easy to detect in lean burn engines usingconventional “J-gap” automotive type spark plugs because the signal isof very low intensity and has great variability, often measured andreferred to as a coefficient of variability (COV). Systems that useconventional “J-gap” automotive type spark plugs to detect an ion signalwill not typically work properly in the case of lean-burn enginesbecause these systems will get a weak post-flame signal or no signal atall. In a preferred form of the present invention, a shielded spark plugin conjunction with the ion circuitry is the ideal choice to measure ionsignals. Thus, a mechanical shield, around the ion probe prevents theinterference of flame related noise with ion sensing elements and helpsimprove the signal-to-noise ratio of the post-flame ion signal detectedby the diagnosis module.

FIG. 5 plots a graphical representation of the post-flame ion signalwithin the ion window versus engine crank angle for different knockmargin conditions. This figure and FIG. 4 are the basis of the ionsignal processing in a diagnosis module, such as the diagnosis module202 of FIG. 2. The time for measuring knock in the ion signal has to bespecified in terms of the engine crank angle degrees. Thus termed“knock-windowing” which helps to reduce the risk of knock misdetection.Knowing the start and stop time of the windowing also makes it possibleto know in which cylinder knocking occurs, so that consideration can betaken during knock control (such as varying the A/F and/or varying thespark timing). Knock control may be done on one cylinder where there isknocking, or it may be done on all cylinders. The controller mustrecognize when to start and stop the knock window in order to get thebest possible knock detection. The window start and stop time depends onengine speed, engine angle and load, and the controller may have acorresponding “look-up” table. In a preferred embodiment, the enginespeed, cylinder-firing order and other engine parameters are provided tothe engine control module prior to knock detection. A prescribed valueof crank angle degree from the start of spark is specified as the startof knock window. In a preferred embodiment, a crank angle is chosen suchthat the chemi-ion peak is avoided from the ion signal analysis. A valueis also specified for the length of the knock window. A typical valuefor the start of the knock window is 20 crank angle degrees from thestart of spark, and the knock window length is approximately 40 crankangle degrees. However, one skilled in the art would recognize thathigher or lower values of knock window start and length may be useddepending on other engine parameters.

Signal processing of an ion signal in the knock window is used tocharacterize the severity of knocking in an engine. In lean-burnengines, the ion signal may be very noisy. Therefore, once thepost-flame ion signal is detected, the acquired signal is filtered usinglow pass filters. The low pass frequency chosen can be a function of theengine speed, engine cylinder geometry and atmospheric conditions. Atypical value of the low pass frequency chosen is 1000 Hz or greater andmay be 2000 Hz in an embodiment. However, higher or lower values of lowpass frequency may be used depending on engine parameters.Alternatively, the signal can be de-noised by convolution withappropriate 1D filters or moving window averaging or weighted cycleaveraging or a combination.

Again referring to FIG. 5, a graphical representation of the multi-cycleaverage of this filtered post-flame ion signal versus engine crank anglefor different knock margins is shown. As defined earlier, incipientknock is indicated by an ion peak location distribution skewed towardadvancing crank angle degrees. The diagnosis module 202 of FIG. 2analyzes the combustion ion waveforms as follows to determine if anincipient knock has occurred. The filtered post-flame ion signal for agiven combustion cycle is analyzed to determine the location of the peakamplitude in terms of crank angle degrees. Then, a multi-cycle movingaverage of the location of the peak amplitude of the filtered post-flameion signal is computed. This multi-cycle moving average is to furtherfilter the ion signal and should not be confused with other discussionsof multi-cycle averages. The number of cycles to average is definedbased upon other ion signal characteristics and engine parameters. Atypical value of the number of combustion cycles to average chosen isbetween 1 to 32 cycles typically 32 cycles. If at least one peak of thispeak location moving average is more advanced than a setpoint threshold,or a target incipient knock rate, then an incipient knock is said tohave occurred.

The efficiency setpoint, peak location setpoint, or simply “setpoint”all refer to a peak location range defined by the normal distribution ofpeak locations for a given engine or operating condition. The efficiencysetpoint adjustment routine shown in FIG. 7 at 716 then adjusts thecenter of the setpoint range for maximum thermal efficiency based onevaluation of between 100 and 900 combustion cycles.

FIG. 6 illustrates a lean burn reciprocating engine 550 according to anembodiment of the present invention. FIG. 6 illustrates an ion sensingelectrode shield 556 that has been found to be beneficial for combustionsensing in lean burn IC engines. In the following discussion the ionsensing probe may be referred to as the spark plug, however, one skilledin the art would recognize that conventional ion sensors other thanspark plugs may be used in the manner illustrated herein for ionsensing.

Prior art ion sensing systems typically use an ion probe or standardspark plugs as integrated ignition device and ion sensor. These sensorstypically protrude into the combustion chamber and contact thecombustion gasses containing ions. The chemical process of combustioncreates many species of ions or charged particles. These ions may bevisualized as a cloud 560 in the combustion chamber containing chargedatoms and molecules of the combustion gasses. Applying a bias voltagecreates an electric field between a sense electrode and a groundelectrode i.e. (the entire combustion chamber). This electric fieldthereby causes ion movement between the electrodes. The movement ofcharged particles or more precisely the movement of electrons is thedefinition of current flow.

Within the cylinder, the ion density in the combustion chamber is afunction of the average temperature and pressure of the combustiongasses. This property of the combustion gas ions is useful for enginecontrol systems. As the cylinder pressure increases, so does themeasured ion current. Thus a peak in the ion signal correlating to thepeak of cylinder pressure is observed, as has been discussed. Thishappens during the thermal ion current phase, and the peak is referredto as the second hump of the ion signal, that is, peak 358 asillustrated in FIG. 4 and also shown in FIG. 5.

However, typical ion current waveforms measured with J-gap or othertraditional spark plug in a lean burn engine deviate from the idealpressure signal considerably. The ion current waveform exhibits largeamounts of “combustion noise”. This noise appears as random increases ordecreases in the ion current throughout the duration of the ion sensewindow. If this noise is great enough it may appear that portions of theion current curve are actually missing, thus creating difficulty indetecting the desired combustion properties.

In normal operation the air, fuel, and combustion gasses flow into andout of the combustion chamber at high rates. It is well known that theflow of these gasses causes turbulence or swirl in the combustionchamber during the combustion process. This turbulence causes thecombustion gasses in the combustion camber to moving past the ion senseelectrode 554 resulting in ion current instability or noise.

A shield or mechanical filter of the ion sensing electrode 554 reducesthe ion current instability which may result from the movement of thecombustion gasses 560. Accordingly, fewer momentary perturbations of theion current and improved detection of the desired combustion parametersfrom the ion current waveform may result. One skilled in the art wouldrecognize that the exact size and shape of the shield is dependent onthe engine flow dynamics and may vary substantially with engine type.

Referring again to FIG. 6, an ion sensor shield 556 is shown. Note thatthe ion sense electrode 554 is illustrated by a protrusion to indicatethat any type of spark plug gap may be shielded in by this method.Possible spark gap types include J-gap, fine wire, surface gap, ormassive electrode. The sensor shield 556 completely encloses the ionsensor electrode 554 and one or more openings 558 are provided to allowthe flow of combustion gasses to contact the ion sensor electrode 554.In this fashion the shield 556 prevents the turbulent flow of combustiongasses 560 from contacting the sensor electrode 554 directly. However,the shield 556 nevertheless allows combustion gasses more representativeof the average cylinder ion density, and thereby cylinder pressure, tocontact and be measured by the electrode 554. The result is a smoothedion waveform that more closely correlates to cylinder pressure andenables improved combustion parameter detection.

The following FIGS. 7-13 illustrate control logic for a single cylinder.However, one skilled in the art would recognize that a control systemfor, for instance, a 16 cylinder engine would maintain essentially 16separate copies of ion data and control parameters, one for eachcylinder.

FIG. 7 illustrates a control algorithm 700 according to an embodiment ofthe present invention. An inside control, or fast response loop 702provides incipient knock detection and prevention based on peak ionreadings occurring in a short number of engine cycles. The fast responseloop 702, based on ion control, typically obtains and processes ionsignal peaks at 704, evaluates the peak location versus setpoint at 706,and quantifies the knock and generates detection outputs at 708. Oncedetection outputs are determined at 708, control action is taken at 710.Furthermore, however, the detection outputs at 708 are also used in asecond loop 712 to evaluate incipient knock at 714 and adjust theefficiency setpoint at 716.

FIG. 8 illustrates details of the general control algorithm 700 asillustrated in FIG. 7. In this embodiment, algorithm 746 includes aninside control, or fast response loop 748 that provides incipient knockdetection and prevention based on peak ion readings occurring in a shortnumber of engine cycles, typically 1 to 15 engine cycles (<1 second)while an outside or slower control loop 750 maintains an efficientsetpoint for the present fuel quality or operating conditions andresponds within 100 to 900 engine cycles (typically >1 minute). Thefirst step in the process 752 is to acquire ion waveform data for thecurrent firing cylinder. The knock window for the given parameters isidentified at 754, and the data is the filtered 756, cycle averaged 758,and the ion peak location is determined at 760.

At 762 the peak location versus setpoint evaluation block performs thesimple task of comparing the current average peak location with theupper and lower setpoint limits. If the current peak location is moreadvanced than the setpoint advance limit (SP_AL), the function generatesa flag signaling that the current peak is advanced, or an incipientknock. If the current peak is within the limits a flag signals then nocontrol action is required. If the peak more retarded than the setpointretard limit (SP_RL) a flag states that the peak is retarded.

At 764, if knock has been detected, the knock is quantified andcombustion parameter outputs are adjusted to prevent knock or maintainion peak location within the setpoint range. At 764, some history andstatistics of the previous combustion cycles is maintained, which isused to determine the proper output parameters to an engine controlmodule, such as the engine control module 206 of FIG. 2.

Control action is taken at 766 wherein the engine and/or ignitioncontrol system response to the current combustion cycle is determined.Appropriate responses can be programmed for a particular engine. Forexample, the response to the detection of an incipient knock could be toretard the timing 0.5 degrees for a typical engine.

At 768 the efficiency setpoint is evaluated. This block maintains alarge history of up to 900 or more combustion cycles and determines thenumber of incipient knock combustion cycles detected for the currentcylinder in the last 100 to 900 cycles. The percent incipient combustioncycles is then compared to a desired target. An output is generatedsignaling that the level of incipient knock is either within range orout of range requiring an adjustment to the ion peak location setpoint.

Block 770 provides an ion peak setpoint adjustment if necessary. Ideallythe maximum efficiency point for a given fuel quality would be obtainedwhen the ion peak setpoint is at the edge of incipient knock. If thesetpoint is at this location, very limited numbers of incipient knockswill be detected and the fast control loop 748 will make very fewadjustments to maintain the peak locations within the setpoint range.However, if for instance the fuel quality drops, the fast control loop748 will detect incipient knock at an increased rate as it attempts tomaintain an ion peak at too advanced a crank angle for the fuel quality.If this condition persists, the slower efficiency control loop 750 willretard the setpoint appropriately to maintain the target incipient knockrate.

Referring still to FIG. 8, an A/D converter samples the ion current at aminimum sample rate of 10 kHz (typically 50 kHz) and filters at 756 toreduce noise, with a two sample moving average filter as they are storedin a buffer. This buffered combustion cycle waveform then updates amulti cycle average ion waveform. The multi-cycle average can be from 1to 32 cycles (typically 32), and is updated at 758. The averagedwaveform is then processed at 760 to determine the ion peak location incrank angle degrees. The ion peak location is then passed at 762 to thepeak evaluation versus setpoint function.

FIG. 9 illustrates details of the peak location evaluation function 762of FIG. 8. Of interest here is the optional proportional retard function778. If this function 778 is used, and the current peak is advanced, thedifference “delta” between the SP_AL and the current peak location (PL)is calculated. This value can be used as a multiplier of the retardtiming step implemented by the engine control or ignition system. Inthis way a peak location that is greatly advanced, most likely a moresevere incipient knock, will cause a larger retard timing step.

Furthermore, FIG. 9 includes an optional peak magnitude qualifier 772.In most cases incipient knock detection can be effectively determinedsimply by peak location. However, some applications with large peaklocation COV (coefficient of variation), due to less stable combustion,may experience false knock detections and may benefit from the magnitudequalifier function 772. This function 772 compares the magnitude of thecurrent cycle peak to a multi-cycle moving average of the peakmagnitude. If the current peak is both more advanced than the SP_AL, andlarger in magnitude than the multi-cycle moving average, it can be moreaccurately determined to be a knocking cycle. As discussed with respectto FIG. 13, the peak location (PL) is assessed against the SP_AL at 774,and also against the SP_RL at 776, to determine at 777 whether toadvance or retard the timing.

Referring back to FIG. 8, the knock is quantified at 764. This function764 receives its input from the peak location versus setpoint evaluationblock 762 and generates combustion parameter outputs includingincipient, moderate, or heavy knock detection. Additional detail forthis block is shown in FIG. 10. This block 764 uses an event countingmethod to determine the correct combustion parameters and recommendedcontrol action. In the case of several combustion events with advancedpeaks, the advanced peak detection for each cycle will cause theadvance_peak_counter (APC) to be increased by one count.

In a preferred embodiment and referring to FIG. 10, at 792 if the countis less than 2, an isolated incipient knock is ignored as being in thenormal noise level. For other engines with more stable combustion thedetection can be triggered on the first advanced peak location. Ineither case when the desired count is reached the engine controller'sresponse would typically be to retard the timing by the preset retardstep. Typically detection of a few incipient knock cycles would trip theignition control to retard the timing sufficiently to prevent knock.However, during severe operating point or fuel transients more than 2advanced peaks in sequence may be detected. In the case that 5 or 10advanced peaks are detected, at 794 and 796 respectively, a moderate orheavy knock can be flagged, thus signaling the engine controller toretard the timing by a larger degree step. Finally in the unlikely eventthat the engine controller retard action does not eliminate knock eventsin within the allowable system response time, at 798 a shutdown (ormajor system fault) recommendation can be generated. The enginecontroller can then take action to protect the engine from damage. Inany case when a peak location is within the setpoint range the APCcounter is reset and armed to detect the next knock event.

Similarly, at 800, if the peak location is retarded theretarded_peak_counter (RPC) is increased by one count at 802. In thiscase the timing is advanced very slowly to allow the cylindercomponents, predominantly the spark plug, to cool after one or moreknock events causing the timing to be retarded. If the timing isadvanced too rapidly after knock events, additional heating of thecylinder can cause more severe knock and a shutdown of the engine may berequired to prevent engine damage. Therefore, at 804 a delay countthreshold is implemented that requires typically 20 retarded peakdetections before signaling the engine controller to advance the timingby a predetermined advance step. Again if a peak location is within thesetpoint range the RPC counter is reset and armed to start counting thenext sequence of retarded peaks. Counters are reset at 806 underconditions where the peak is neither advanced nor retarded. Enginecombustion parameters are output to the controller at 807.

FIG. 11 illustrates a detailed set of recommended engine and ignitioncontroller responses 766 that may be taken, based on the quantificationof knock at 764 in FIG. 8. As illustrated, at 820 a typical response toan incipient knock detection is a 0.5 degree timing retard 822. Next, at824 if knock is deemed to be moderate, a typical response is to retardthe timing by 1.0 degree at 826. At 828 if knock is deemed to be heavy,a typical response at 830 is to retard the timing by 5.0 degrees.Finally, if shutdown knock is detected at 832, a typical response at 834is to initiate a shutdown procedure. Adjustments to the controller aremade at 839.

In operation, if knock has been detected due to a fuel qualitytransient, but the fuel later returns to the previous quality, thetiming may temporarily be retarded more than required for the currentconditions. This would result at 836 in a series of retarded peaklocations. The typical engine control response to retarded peaklocations is to advance the timing in 0.2 degree steps at 838 every 20retarded peaks. This slow response allows the cylinder components tocool after knock events preventing additional knocking cycles. Typicallythe timing returns to the setpoint within several seconds, to around aminute, depending on how many degrees the timing was retarded during theknock event. A large knock event may retard the timing 5 to 10 degrees.

However, in most cases moderate, heavy, or shutdown knock events willnot be detected. The responses illustrated 824, 828, 832, and 836 arepresent only to provide a proportional response to large knock eventsdue to extreme transients, or a shutdown signal in the event ofuncontrollable detonation or knock typical of pre-ignition. Pre-ignitionis a condition where the internal temperatures of the cylinder are highenough that the spark is no longer required to ignite the mixture. Thus,the cylinder surface temperatures, typically the plug electrodes, arehigh enough that under the pressure of the combustion strokeauto-ignition (or dieseling) occurs. This is a thermal runaway conditionwhere the early ignition of the mixture causes the cylinder temperatureto continue to rise causing even earlier auto-ignition. Under theseconditions spark timing retard has little or no effect and engineshutdown, or fuel cutoff, is the only remedy.

FIG. 12 illustrates details of evaluation of the assessment of thepercent of incipient knock, as illustrated at 768 in FIG. 8. Percentincipient knock is also referred to as incipient knock rate, orincipient knock frequency rate. Maximum efficiency is typically at themost advanced timing possible without incurring knock, or a slightlyretarded offset from this point. This varies slightly with the type ofengine. According to an embodiment of the present invention, the ionpeak location setpoint is adjusted by the efficiency incipient knockevaluation routine in an attempt to maintain the ideal ignition timingfor a given fuel quality or operating condition. This functional block768 maintains a knock detection history of between 100 to 900 combustioncycles. Any knocking cycle classified as an incipient knock or greaterat 850 is logged in the buffer or FIFO (first-in first-out shiftregister) at 852, and any non-knocking cycles are also logged at 854. Atarget incipient knock frequency rate of only a few cycles per 100 isdesirable to place the setpoint at the edge of incipient knock. Thenumber of incipient knocking cycles in the last 100 to 900 engine cyclesis thus compared to this target level. At 856, if the frequency rate ofincipient knock events is below the target value maximum efficiency maynot be obtained, and at 857 the setpoint adjustment routine is signaledto advance the setpoint to better take advantage the higher fuel qualitycurrently available. At 858, if the frequency rate of incipient knockevents is above the desired target, the fuel quality cannot enableoperation at such an advanced setpoint. Therefore, at 857 the setpointadjustment routine is signaled to retard the setpoint. As illustrated,no action is required if the incipient knock frequency rate is withinthe target range of 5 to 10%.

Details of setpoint adjustment routine 857 from FIG. 12 are illustratedin FIG. 13. This function 857 simply receives an “Above Target”indication 870 or “Below Target” indication 872 from the efficiencyincipient knock evaluation block 768 of FIG. 8, and retards 874 oradvances 876 the setpoint by a predetermine increment. These step sizes,or gains, may be tuned for best performance and efficiency for the gasquality fluctuations typical for a given installation site. The qualityof natural gas may vary greatly and the ranges over which the quality ormethane number change and the rate of change are not generally known.Therefore, some empirical tuning of the sensitivity or target range, andthe control response time or timing step size for a given installationsite may be required. Note that hard limits are used for the maximumsetpoint advance and retard (0.1 degree in both cases, as an example).This prevents operating the engine outside of reasonable limits in thecase that unreasonable operating conditions exist.

It is important to note that there are both advance, and retard,setpoint markers or thresholds that are used for the actual peaklocation evaluation as shown in FIG. 9 at 774 and 776. Also, FIG. 12shows similar “below target” and “above target” incipient knock ratethresholds. This provides a dead band wherein the setpoint is withinrange at 878 in FIG. 13 and for which the control takes no action. Thissimply provides a band around the setpoint such that normal peaklocation jitter (or normal peak location noise distribution as describedpreviously in defining incipient knock detection and describing signalprocessing) does not cause excessive control loop adjustments. Oneskilled in the art would recognize that the width of the setpoint bandmay be slightly different from one engine type to another and 0.1 degreeincrements of adjustment shown in FIG. 13, at 874 and 876, are made forpurposes of illustration only.

In an alternate embodiment of the present invention, it may be desirableto operate at an offset retarded from the incipient knock threshold.This is equivalent to using wider setpoint range such that the limitcycle behavior effectively allows the average setpoint to be maintainedat the desired offset from the incipient knock threshold. Once theincipient knock threshold has been reached using a larger retard step toplace the setpoint at the desired retard offset.

In another embodiment, this same offset operation may be accomplished asfollows. In this case once the incipient knock threshold is reached thesetpoint would be retarded by a preset offset amount. The setpoint wouldthen remain in an open loop (or fixed) mode for an acceptable timeperiod. An automatic calibration cycle would then be required at regularintervals to ensure that the setpoint is maintained at the desiredoffset from the incipient knock threshold. The calibration cycle wouldrequire temporary closed loop operation of the setpoint routine to makea short excursion advancing the setpoint to the incipient knockthreshold. Upon determining the incipient knock threshold, the setpointwould again be retarded by the desired offset and the setpoint wouldagain remain in open loop or fixed for an acceptable period of time.

After the completion of the final functional block, the control systemrepeats the same control flow for the next cylinder in the firing order.Then after completing evaluation of all cylinders for a particularengine, the process repeats with the first cylinder in the firing order.Ion data and control parameters are maintained for each individualcylinder of the engine. The engine controller then evaluates combustionparameters for all cylinders of the engine and initiates individualcylinder or global control actions related to a plurality, or even all,cylinders. Also, note that some control actions may be global (orpertaining to all cylinders) not limited to a severe knock conditionrequiring engine shutdown is one such example.

The aforementioned control system is the result of a simple attempt tocreate a control algorithm capable of protecting an engine andmaintaining maximum efficiency. The initial approach has been to use anonlinear controller with a setpoint dead band and higher retard gainthan advance gain. This approach provided good proof of concept testperformance. However, by its nature it would likely exhibit a limitcycle whereby performance may not be optimal. Therefore, there is noattempt to limit or preclude other control algorithm approaches. Alinear control algorithm with no dead band and equal retard and advancegains may also provide satisfactory or improved performance.

The present invention has been described with respect to the use of aspark plug as an ion sensor in a reciprocating internal combustionengine to identify and operate at the onset of incipient knock, whereinthe spark plug has an electrode that serves for both purposes ofproviding spark and sensing an ion current during the combustionprocess. One skilled in the art will appreciate that ion sensors ingeneral will be applicable in a number of applications according toembodiments of the present invention. In this regard, it is appreciatedthat various sensors or plug designs can readily be optimized for agiven application without departing from the spirit and scope of theappending claims.

It is understood that for certain engine applications a shielded sparkplug in unnecessary to practice aspects of this invention. One skilledin the art may recognize that an ion current may be detected by anelectrode of a spark plug without having a shield and, as such,detection of incipient knock with a conventional and exposed electrodemay provide acceptable performance.

Therefore, in accordance with one embodiment of the present invention, asystem for controlling knock in a lean burn internal combustion (IC)engine includes a spark plug having an electrode, and an electricalcircuit configured to provide a first voltage to the electrode anddetect an ion current during a thermal-ionization phase of thecombustion process, and provide a second voltage to the electrode tocreate a spark and initiate a combustion process within a combustionchamber. The engine includes a controller configured to monitor the ioncurrent for a knock condition that includes at least an incipient knockcondition, determine a spark crank angle timing of the IC engine wherethe incipient knock occurs, and adjust the spark timing of the IC engineto operate at a crank angle that does not exceed a threshold levelbeyond an incipient knock set point.

In accordance with another embodiment of the present invention, a methodfor combustion feedback control of a lean-burn reciprocating internalcombustion engine using ion signals includes the steps of positioning aspark plug having an electrode, the spark plug positioned at leastpartially within a combustion chamber of the engine, initiatingcombustion within the combustion chamber by providing a voltage to theelectrode, measuring an ion current using the electrode duringcombustion, and adjusting spark timing of the IC engine to achieve andmaintain maximum thermal efficiency by operating at reduced knockmargin.

According to another embodiment of the present invention, a closed-loopcontroller for a spark-ignition internal combustion (IC) engine includesa control to detect an ion current within a combustion chamber of the ICengine using an electrode of a spark plug, determine a desired crankangle for spark timing from the ion current wherein incipient knockbegins to occur, and continually monitor and adjust spark timing of theIC engine to operate at or below inception of incipient knock.

According to still another embodiment of the present invention, a systemfor controlling knock in a lean burn internal combustion (IC) engineincludes an electrical circuit and a controller. The electrical circuitis configured to provide a first voltage to an igniter and detect an ioncurrent during a thermal-ionization phase of the combustion process, andprovide a second voltage to the igniter to create a spark and initiate acombustion process within a combustion chamber. The controller isconfigured to monitor the ion current for a knock condition thatincludes at least an incipient knock condition, determine the sparkcrank angle of the IC engine where the incipient knock occurs, andadjust spark timing of the IC engine to operate at a crank angle thatdoes not exceed a threshold level beyond inception of incipient knock.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A system for controlling knock in a lean burn internal combustion(IC) engine comprising: a spark plug having an electrode; an electricalcircuit configured to: provide a first voltage to the electrode anddetect an ion current during a thermal-ionization phase of thecombustion process; and provide a second voltage to the electrode tocreate a spark and initiate a combustion process within a combustionchamber; and a controller configured to: monitor the ion current duringa post-flame ionization; compute a multi-cycle moving average of alocation of peak amplitudes of the ion current during the post-flameionization; determine a spark crank angle timing of the IC engine whenat least one peak amplitude of the moving average is more advanced thana setpoint threshold; and adjust the spark crank angle timing of the ICengine based on the determination.
 2. The system of claim 1 wherein,when the at least one peak amplitude is more advanced than the setpointthreshold, a knock condition is indicated that includes at least one ofmoderate knock, heavy knock, and shutdown knock.
 3. The system of claim2 wherein the controller is further configured to retard timing if theknock condition is identified.
 4. The system of claim 2 wherein themoving average is determined based on a number of advanced peaks thatoccur sequentially or statistically.
 5. The system of claim 2 whereinthe moving average is determined based on a history of previouscombustion cycles.
 6. The system of claim 1 wherein an incipient knockis identified by at least one or more sequential advanced peakamplitudes.
 7. The system of claim 1 wherein an incipient knock isidentified by at least one or more statistically-based or statisticallydetermined advanced peak amplitudes.
 8. The system of claim 1 whereinthe controller adjusts timing based on an algorithm having both a fastresponse loop and a slow response loop.
 9. The system of claim 8 whereinthe fast response loop includes an assessment of an incipient knockusing between 1 and 15 engine cycles, and the slow response loopincludes an assessment of incipient knock using between 100 and 900engine cycles.
 10. The system of claim 1 further comprising a shieldsurrounding the electrode that acts as a mechanical filter to shield theelectrode from combustion noise or turbulence.
 11. The system of claim10 wherein the shield is cylindrical in shape having an open endopposite the electrode.
 12. A method for combustion feedback control ofa lean-burn reciprocating internal combustion (IC) engine using ionsignals, the method comprising the steps of: positioning a spark plughaving an electrode, the spark plug positioned at least partially withina combustion chamber of the engine; positioning a shield about theelectrode, the shield having an aperture therein and configured toenclose but not contact the electrode; initiating combustion within thecombustion chamber by providing a voltage to the electrode; measuring anion current using the electrode during combustion, the ion currentresulting from combustion gasses that pass through the aperture; andadjusting spark timing of the IC engine to achieve and maintain maximumthermal efficiency by operating at reduced knock margin.
 13. The methodof claim 12 wherein the knock margin is indicated by an incipient knockcondition identified as having at least one or more sequential advancedpeaks.
 14. The method of claim 12 wherein the knock margin is indicatedby an incipient knock condition identified as having at least one ormore statistically advanced peaks.
 15. The method of claim 12 whereinthe knock margin is indicated by an incipient knock condition identifiedbased on a history of previous combustion cycles.
 16. The method ofclaim 12 wherein the spark timing is adjusted proportionally to thedifference between a peak location and a set point.
 17. The method ofclaim 12 further comprising measuring the ion current during athermal-ionization phase of the combustion process.
 18. The method ofclaim 12 further comprising evaluating the incipient knock rate of theengine and adjusting the spark timing to maximize thermal efficiency atreduced knock margin.
 19. The method of claim 18 further comprisingevaluating the incipient knock rate over a suitable range of enginecycles to maximize thermal efficiency at reduced knock margin.
 20. Aclosed-loop controller for a spark-ignition internal combustion (IC)engine comprising: a control to: detect an ion current within acombustion chamber of the IC engine using an electrode of a spark plug;compute a multi-cycle moving average of a location of peak amplitudes offiltered post-flame ion currents; determine a desired crank angle forspark timing from the ion current wherein incipient knock begins tooccur, based on the computed multi-cycle moving average; and continuallymonitor and adjust spark timing of the IC engine to operate at or belowinception of incipient knock.
 21. The controller of claim 20 wherein thecontrol further initiates combustion within the IC engine using theelectrode.
 22. The controller of claim 20 wherein the control identifiesincipient knock when one or more advanced peaks occur sequentially. 23.The controller of claim 20 wherein the control identifies incipientknock when one or more advanced peaks occur statistically.
 24. Thecontroller of claim 20 wherein the control identifies incipient knockbased on a history of previous combustion cycles.
 25. The controller ofclaim 20 wherein the control identifies the IC engine as having one ofan incipient, moderate, heavy, and shutdown level of knock and adjustingthe timing based on that identification to operate at or below anincipient knock set point.
 26. A system for controlling knock in a leanburn internal combustion (IC) engine comprising: an electrical circuitconfigured to: provide a first voltage to an igniter and detect an ioncurrent during a thermal-ionization phase of the combustion process; andprovide a second voltage to the igniter to create a spark and initiate acombustion process within a combustion chamber; and a controllerconfigured to: monitor the ion current for a knock condition thatincludes at least an incipient knock condition; select a low pass filterfrequency as a function of one of an engine speed, an engine cylindergeometry, and an atmospheric condition; filter the ion current using alow pass filter having the determined low pass filter frequency;determine the spark crank angle of the IC engine where the incipientknock occurs; and adjust spark timing of the IC engine to operate at acrank angle that does not exceed a threshold level beyond inception ofincipient knock.